The present invention relates in general to the coating arts and more particularly, to the production of coatings by thermal spray techniques.
Plasma spraying devices and techniques are well known in the art for depositing protective coatings on underlying substrates. One known device is illustrated in U.S. Pat. No. 3,145,287 to Siebein el al entitled "Plasma Flame Generator and Spray Gun." In accordance with the teaching of the Siebein et al patent, a plasma-forming gas forms a sheath around an electric arc, constricts and extends the arc part way down the nozzle. The gas is converted to a plasma state and leaves the arc and nozzle as a hot free plasma stream. Powders are injected into the hot free plasma stream and propelled onto the surface of the substrate to be coated.
A prior art device, such as that illustrated by Siebein et al, is employed in the apparatus of the present invention to generate a hot plasma stream and is identified as item 6 of the Drawing.
U.S. Pat. Nos. 3,851,140 to Coucher entitled "Plasma Spray Gun and Method for Applying Coatings on a Substrate" and 3,914,573 to Muehlberger entitled "Coating Heat Softened Particles by Projection in a Plasma Stream of Mach 1 to Mach 3 Velocity" disclose contemporaneous coating technology. Both contemporaneous patents are common with Siebein et al in that coating powders are introduced immediately downstream of the point at which the plasma is generated. Physically, the point of injection in each case is at the downstream end of the anode within which the plasma is generated.
In addition to the Siebein et al and Muehlberger structures, Coucher employs a tubular nozzle downstream of the point of powder injection. According to the Coucher specification heat fusible material is thermally liquified as it contacts the hot plasma and is ejected with the hot plasma through the tubular nozzle.
Although the methods and devices disclosed likely have utility in the coating industry, scientists and engineers continue to search for yet improved coating methods and techniques.